1. Field of the Invention
The present invention relates to a television tuner capable of receiving FM broadcast signals.
2. Description of the Related Art
FIG. 5 illustrates a television tuner capable of receiving FM broadcast signals. A television signal, represented by TV, or an FM broadcast signal, represented by FM, is received by an input tuner circuit 21. The input signal is conditioned by a first mixer circuit 24, a radio frequency (RF) amplifier circuit 22, and an interstage tuner circuit 23.
The first mixer circuit 24 receives a local oscillation signal from a first local oscillator circuit 25. The tuned frequency of the input tuner circuit 21, the interstage tuner circuit 23, and the frequency (local oscillating frequency) of the first local oscillator circuit 25 are controlled by a channel select signal S received by a phase-locked loop (PLL) circuit 26. Generally, the local oscillating frequency is higher than the tuned frequency.
When receiving a television signal, a television intermediate-frequency signal having a video intermediate-frequency, according to US standards, e.g., a video intermediate-frequency of 45.75 MHz, is generated by the first mixer circuit 24. When necessary, an FM broadcast signal (hereinafter referred to as the xe2x80x9cfirst FM IF signalxe2x80x9d) is converted to a television intermediate-frequency signal by the first mixer circuit 24. The television intermediate-frequency signal generated by the first mixer circuit 24 is passed to a subsequent intermediate-frequency circuit (not shown) through a first intermediate-frequency filter 27 for further processing.
The first FM IF signal generated by the first mixer circuit 24 is also received by a second mixer circuit 29 through a second intermediate-frequency filter 28. The second mixer circuit 29 receives a local oscillation signal from a second local oscillator circuit 30. The frequency of the local oscillation signal generated by the second local oscillator circuit 30 is 10.7 MHz higher in frequency than the frequency of the first FM IF signal. Therefore, an FM broadcast signal (hereinafter referred to as the xe2x80x9csecond FM IF signalxe2x80x9d) having a frequency converted to a 10.7 MHz signal is generated by the second mixer circuit 29. The second FM IF signal is received by a demodulator circuit (not shown) through a third intermediate-frequency filter 31.
Accordingly, a television tuner can include two mixer circuits 24 and 29, and two local oscillator circuits 25 and 30 that achieve a two-stage frequency conversion in which an FM broadcast signal is first frequency-converted to a television intermediate-frequency signal before being converted to an intermediate-frequency signal of 10.7 MHz used in a conventional FM receiver. This two-stage structure creates beat disturbances between the two local oscillation signals, which interferes with reception.
A television tuner preferably includes a single mixer circuit and a single local oscillator circuit which provides a simplified layout that is immune to a wide range of interference such as beat disturbances.
In one aspect, a television tuner includes a mixer coupled to a local oscillator circuit. Preferably, the mixer is configured to receive a television signal and an FM broadcast signal. The local oscillator circuit generates a local oscillation signal received by the mixer circuit. When a television signal is received, a television intermediate-frequency signal is generated by the mixer circuit. When an FM broadcast signal is received, an FM intermediate-frequency signal converted to 10.7 MHz is generated by the mixer circuit. Only a single local oscillator circuit is used to obtain a television intermediate-frequency signal and an FM intermediate-frequency signal converted to about 10.7 MHz.
Preferably, the television signal or the FM broadcast signal is received by a tuner circuit that includes a mixer circuit. The local oscillation frequency and the tuned frequency of the tuner circuit can be adjusted when the local oscillation frequency is higher than the tuned frequency of the tuner circuit by a frequency of a television intermediate-frequency signal. In embodiments where the frequency of the local oscillation signal is about 10.7 MHz higher than the frequency of the FM broadcast signal, the tuned frequency may be adjusted so that the difference between the tuned frequency and the frequency of the local oscillation signal is about 10.7 MHz. Therefore, when the television signal is received, a television intermediate-frequency signal is generated by the mixer circuit, and when the FM broadcast signal is received, an FM intermediate-frequency signal having a frequency converted to about 10.7 MHz is generated by the mixer circuit.
In another aspect, the television tuner includes a first varactor diode and the local oscillator circuit includes a second varactor diode. Preferably, the frequency of the local oscillation signal is higher than the tuned frequency of the tuner circuit by a frequency of the television intermediate-frequency signal when a common voltage is applied to the first varactor diode and to the second varactor diode. In applications where the frequency of the local oscillation signal is about 10.7 MHz higher than the frequency of the FM broadcast signal, a higher voltage is applied to the first varactor diode than to the second varactor diode. The difference between the tuned frequency and the frequency of the local oscillation signal may thus be about 10.7 MHz.
The television tuner may include a phase lock loop (PLL) circuit for generating a tuning voltage that is applied to the first varactor diode and to the second varactor diode. The television tuner further includes a voltage adder circuit, and a voltage supply that supplies a positive voltage. Preferably, the tuning voltage is applied to a cathode of the second varactor diode and to an input of the voltage adder circuit. Preferably, a positive voltage is applied to the voltage adder circuit only when an FM broadcast signal is received. The positive voltage is added to the tuning voltage to generate a voltage that is applied to the cathode of the first varactor diode.
Alternatively, the television tuner may include a PLL circuit for generating a tuning voltage that is applied to the first varactor diode and to the second varactor diode and a voltage supply that supplies a negative voltage. Preferably, the tuning voltage is applied to the cathode of the first varactor diode and to the cathode of the second varactor diode, and the negative voltage is applied to an anode of the first varactor diode only when the FM broadcast signal is received. Accordingly, a higher voltage is applied to the first varactor diode.
Preferably, the television signal or the FM broadcast signal is received by a tuner circuit before the signal is conditioned by the mixer circuit. Preferably, the frequency of the local oscillation signal and the tuned frequency of the tuner circuit can be adjusted while the frequency of the local oscillation signal is higher than the tuned frequency of the tuner circuit by a frequency of the television intermediate-frequency signal. In applications where the tuned frequency corresponds to the frequency of the FM broadcast signal, the frequency of the local oscillation signal is adjusted so that the difference between the tuned frequency and the frequency of the local oscillation signal is about 10.7 MHz. Therefore, when the television signal is received, a television intermediate-frequency signal is generated by the mixer circuit. When the FM broadcast signal is received, an FM intermediate-frequency signal having a frequency converted to about 10.7 MHz is generated by the mixer circuit.
Preferably, the television tuner includes a first varactor diode and the local oscillator circuit includes a second varactor diode. Preferably, the frequency of the local oscillation signal is higher than the tuned frequency by the frequency of the television intermediate-frequency signal while a common voltage is applied to the first varactor diode and to the second varactor diode. In applications where the tuned frequency corresponds to the frequency of the FM broadcast signal, a lower voltage is applied to the second varactor diode than to the first varactor diode. The difference between the tuned frequency and the frequency of the local oscillation signal may thus be about 10.7 MHz.
The television tuner may include a PLL circuit for generating a tuning voltage that is applied to the first varactor diode and to the second varactor diode. The television tuner further includes a voltage subtractor circuit and a voltage for supplying a positive voltage. Preferably, the tuning voltage is applied to the cathode of the first varactor diode and to an input of the voltage subtractor circuit. Preferably, a positive voltage is applied to the voltage subtractor circuit only when the FM broadcast signal is received. The voltage subtractor circuit subtracts the positive voltage from the tuning voltage, to generate a voltage that is applied to the cathode of the second varactor diode.
Alternatively, the television tuner may include a PLL circuit for generating a tuning voltage that is applied to the first varactor diode and to the second varactor diode, and a voltage supply that supplies a positive voltage. Preferably, the tuning voltage is applied to the cathode of the first varactor diode and to the cathode of the second varactor diode, and the positive voltage is applied to the anode of the second varactor diode only when the FM broadcast signal is received. Accordingly, a lower voltage is applied to the second varactor diode.